He has a line in chaff (5)
I believe the answer is:
brian
'he' is the definition.
'a line in chaff' is the wordplay.
'a line' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'chaff' becomes 'bran' (bran is a kind of chaff).
'i' inserted inside 'bran' is 'BRIAN'.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for brian that I've seen before include "-- Blessed, actor" , "-- Blessed; -- Eno" , "He had a 1979 film "Life"!" , "snail animated" , "intellect" .)